A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., including part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning” direction) while synchronously scanning the substrate parallel or anti parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to monitor the lithographic process, it is often necessary to measure parameters of a patterned substrate, such as an overlay error between successive layers formed in or on the substrate. Various techniques exist for making measurements of microscopic structures formed on substrates by lithographic processes, including the use of scanning electron microscopes and various specialized tools. In particular, one form of specialized inspection tool is a scatterometer, which directs a beam of radiation onto a target on the surface of the substrate and measured properties of the scattered or reflected beam. By comparing the properties of the beam before and after it has been reflected (or scattered) by the substrate, the properties of the substrate can be determined. For example, the properties of the reflected beam can be compared with data stored in a library of known measurements associated with known substrate properties.
In general, there are two classes of scatterometers: spectroscopic scatterometers and angularly-resolved scatterometers. Spectroscopic scatterometers direct a broadband radiation beam onto a substrate and measure a spectrum (i.e., intensity as a function of wavelength) of the radiation scattered into a particular narrow angular range. Angularly-resolved scatterometers use a monochromatic radiation beam and measure the intensity of the scattered radiation as a function of angle.
As mentioned above, it is important to ensure that successive layers of products that are printed onto the substrate are aligned with each other. For example, if a single layer of an IC were misaligned, or were to have an overlay error, one portion of the IC could be electrically connected to a different portion of the IC, thus creating an electrical contact that was neither intended nor desired.
Specialized overlay targets are often used to measure overlay and thus, determine whether there are overlay errors on the substrate. These specialized overlay targets are generally printed in non-product areas of the substrate (e.g., areas not containing structures that will eventually be part of the IC or other product). Such non-product areas include scribe lanes, which exist between fields of a substrate and which will be sawn to separate the products once the substrate is fully exposed.
Overlay targets generally include gratings that are made up of arrays of printed bars. Such gratings affect radiation that is reflected from them in predictable ways and variations in the reflected radiation are relatively simple to observe. Overlay targets are generally are printed on a substrate in layers in a manner similar to as the printing of other substrate features. A relative position of one layer of a grating with respect to a successive layer of the same grating is measurable using a scatterometer, as described above.
Overlay errors are rarely constant over an entire substrate and may vary from field to field. In order to monitor this variation, each field is generally associated with several overlay targets. Existing overlay measuring systems often required several targets per field (e.g., four to six targets per field). As there are generally of the order of 100 fields per substrate, the substrate may include several hundred overlay targets that must be printed and measured, thereby consuming time, space and resources.
Further, existing techniques for measuring overlay error generally assume that a simple, linear relationship exists between the overlay and an asymmetry measured by a scatterometer (e.g., a measured difference between the two gratings). Such techniques leverage the assumed linear relationship to reduce both the number of targets within each field of the substrate and the number of overlay measurements made within each target. It is desirable to have as few targets as possible so that valuable “real estate” is not used up, particularly in the scribe lanes of the substrate. The real estate is valuable because there are several different types of targets (or marks) that are desired to be put into the scribe lanes, but the scribe lanes are also preferably made as small as possible so that as little as possible of the substrate surface is wasted (i.e., the surface is not used for the product which can eventually be sold).
The assumption of a linear relationship is valid to a certain extent, e.g., as long as the asymmetry and overlay values are both very close to zero, as depicted in FIG. 11. This is because the relationship of asymmetry with overlay is in fact generally accepted to be sinusoidal and as such, the linear relationship will only work for overlay numbers of less than about a quarter of the pitch (p) of the grating in each direction. However, the assumption of the linear relationship is not accurate for larger overlay errors, nor is it particularly accurate even for small overlay errors, as the curve of the sinusoidal relationship is not taken into account.